Tracking ships using software-defined radio (SDR)

When we first started hearing about software-defined radio hacks (which often use USB dongles that ring it at under $20) we didn’t fully grasp the scope of that flexibility. But now we’ve seen several real-life examples that drive the concept home. For instance, did you know that SDR can be used to track ships? Ships large and small are required by may countries to use an Automatic Identification System (AIS) transponder. The protocol was originally developed to prevent collisions on large ships, but when the cost of the hardware became affordable the system was also brought to smaller vessels.

[Carl] wrote in to share his project (which is linked above). Just like the police scanner project from April this makes use of RTL-SDR in the form of a TV tuner dongle. He uses the SDRSharp software along with a Yagi-UDA. The captured data is then decoded and plotted on a map using ShipPlotter.

I’ve done a lot with locally received ~160MHz AIS ( see => http://www.manuka.orconhosting.net.nz/ais.htm ), & point out that ranges (even with a sensitive VHF receiver, well elevated antenna and near line of sight coverage) are usually only to 10-20km. The earth’s curvature sets the usual limit,although exceptionally reception may extend 100s of km under tropo ducting. This SDR based reception hence will normally be just to very local vessels- I’ve found PC soundcard based AISMon ( free => http://www.navmonpc.com/ ) very suitable for such work. It also includes simple charts.

Marine Traffic’s web feed at http://www.marinetraffic.com/ais/ uses superior shore side receiving stations (often over lapping) & detailed vessel info. As vessels travel at just ~15 knots, display latency is negligible. All up it’s really the best AIS info option for many users living near ports!

Another source of near-real-time AIS data is on the web at http://www.aprs.fi . That site is primarily intended for APRS, a ham radio system that allows anyone with a ham license to send out position reports (as well as other types of data) on ham radio frequencies. AIS data for ships is sent via a different protocol on different frequencies, but both sources of data are collected and displayed together overlayed via google maps on that web site.

That website relies on individuals to set up and maintain the radio receivers which receive and decode the AIS data, performing the same function as this HaD entry’s SDR. If you live near the coast in an area not well covered by the aprs.fi website, you could set up a receiver similar to today’s HaD entry, and pipe its output to aprs.fi. That would not only let you see the AIS data for yourself, but also share it with the world.

Absolutely the CG uses AIS. They want to avoid collisions as much as any other vessels!

To see an example, go to http://www.aprs.fi and in the “track callsign” type the name of a Coast Guard vessel. Some of the many possible names to try: “CG ASPEN”, “CG ACTIVE”, “CG MAPLE”, “CG HICKORY”, “CG ALDER”.

I’m sure they do! It’s globally a legal requirement for all vessels above 300 tons. However the transponders are quite cheap & even small harbour ferries, inland waterway,larger yachts & pleasure craft etc use it (typically as AIS “B”). As transponders draw only a few Watts of power, the electrical load is easy – “Poor mans radar” I heard one boatie say.